The relative potency of standardized grass extracts (orchard, redtop, Bermuda, perennial rye, and timothy) with respect to their respective FDA reference extracts were estimated by both in vivo PLST and in vitro RAST. The concordance between PLST and RAST relative potency was found to be 90.6%. For each grass except timothy, the PLST and RAST relative potency estimates fell within their respective 95% confidence interval demonstrating equipotence by each assay. For timothy grass, the PLST relative potency was just below the 95% lower limit while the RAST relative potency was just above the 95% lower limit. Nonetheless, the concordance of relative potency for timothy between PLST and RAST was 92%. There was no statistically significant difference found between the RAST and PLST relative potency estimates for all the grasses tested (p=0.24). These data indicate that RAST relative potency estimates for standardized grass extracts are highly predictive of their in vivo relative potency.